Solids are held in the arm due to the contraction of muscles and the coordination of joints in the arm. When the muscles in the arm contract, they exert force on the bones and joints, allowing the arm to stabilize and hold onto the solid object. The joints in the arm provide the necessary flexibility and range of motion to adjust the position and grip on the solid object as needed. Additionally, sensory receptors in the skin and muscles of the arm provide feedback to the brain about the pressure and position of the solid object, helping to maintain a secure hold.
